Engine-out emission of carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) depends on the air-to-fuel ratio (A/F), defined by equation (1):A/F=mass of air consumed by the engine/mass of fuel consumed by the engine  (1).The A/F where there is just enough air to complete the combustion of all hydrocarbons in the fuel is known as stoichiometry, and is at 14.7 in gasoline engines. If the A/F is below this value, then the engine operates under excess fuel conditions, leading to incomplete fuel combustion. The exhaust gas will then contain more reducing reactants (CO, HC) than oxidising reactants (O2, NOx), and is called rich. If the A/F exceeds 14.7, then the engine operates under excess air conditions, giving rise to an exhaust gas that contains more oxidising reactants than reducing reactants, and the exhaust gas is called lean.
A common way of classifying the engine-out exhaust gas composition is the lambda (λ) value, defined by equation (2):λ=actual engine A/F/stoichiometric engine A/F  (2)From equation (2) it will be seen that when the exhaust gas composition is lean, λ≧1, and when the exhaust gas composition is rich, 1≧λ.
In order to control NOx in exhaust gases from lean-burn gasoline engines, there has been devised a NOx absorber/catalyst which stores NOx, e.g. as nitrate, when an engine is running lean. In a stoichiometric or rich environment, the nitrate is understood to be thermodynamically unstable, and the stored NOx is released and is reduced by the reducing species present in the exhaust gas. This NOx absorber/catalyst is commonly called a NOx-trap. By periodically controlling the engine to run stoichiometrically or rich, stored NOx is reduced and the NOx-trap is regenerated.
A typical NOx-trap formulation includes a catalytic oxidation component, such as platinum, a NOx-storage component, such as barium, and a reduction catalyst e.g. rhodium. One mechanism commonly given for NOx-storage during lean engine operation for this formulation is: (i) NO+½O2→NO2; and (ii) BaO+2NO2+½O2→Ba(NO3)2. In the first step, the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2 by the storage material in the form of an inorganic nitrate.
When the engine runs under rich conditions or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2 according to equation (iii) below. Under rich conditions, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst. (iii) Ba(NO3)2→BaO+2NO+{fraction (3/2)}O2 or Ba(NO3)2→BaO+2NO2+½O2; and (iv) NO+CO→½N2+CO2 (and other reactions). In the reactions of (i)-(iv) above the reactive barium species is given as the oxide. However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The above reaction schemes can be adapted accordingly for species of barium other than the oxide.
Using sophisticated engine management techniques and known fuel injection components such as common rail, it is now becoming possible to adopt NOx-trap technology into the exhaust treatment systems for diesel engines. See, for example, EP-A-0758713 described below.